Preservative gas generating device

ABSTRACT

The invention provides a preservative gas generating device which has a carrier sheet ( 6 ); a layer of a matrix forming material having a preservative gas generating compound dispersed therein ( 8 ), the layer of matrix forming material being adapted to permit the generation of a preservative gas by the preservative gas generating compound and to permit the gas to permeate out of the matrix formed by the material, the matrix forming material being adhered to the carrier sheet ( 6 ) in the form of a layer covering an area of the carrier sheet so as to form at least one discrete zone ( 8 ) with a peripheral area of the carrier sheet surrounding the discrete zone(s) being substantially free of the matrix forming material; and a cover sheet ( 10 ) secured to said peripheral area to enclose the layer of matrix forming material between the carrier sheet and the cover sheet; and wherein at least one of the carrier sheet and the cover sheet are permeable to said preservative gas. The invention also provides a method for manufacturing the preservative gas generating device.

FIELD OF THE INVENTION

[0001] This invention relates to preservative gas generating deviceswidely used in the fruit packaging industry in order to preserve thequality of fruit for extended storage and shelf-life periods, thepreservative gas generating devices typically being packaged in aplastic bag together with the fruit in a box. The technique isparticularly widely used in the packaging and distribution of tablegrapes.

[0002] More particularly, but not exclusively, the invention relates tosulphur dioxide (S0₂) gas generating devices of well-known general typewherein a chemical compound which is carried, usually within an envelopeor on a sheet of paper or the like, reacts with moisture in the plasticbag to produce sulphur dioxide gas which exhibits the preservativefunction. One chemical compound widely used for this purpose is sodiummetabisulphite.

BACKGROUND TO THE INVENTION

[0003] Numerous different types of in-package sulphur dioxide generatingdevices have been proposed and various of these are in commercial use.Whilst existing sulphur dioxide generating devices are effective to someextent there are deficiencies in at least some respects.

[0004] The first general difficulty is to obtain an effective balancebetween the amount of sulphur dioxide produced; the rate at which it isreleased; and the length of time during which satisfactory release ofsulphur dioxide can be achieved. Many commercially available sulphurdioxide generating devices give off a satisfactory amount of sulphurdioxide gas in the early stages but the S0₂ production decreases to anunsatisfactorily low level too soon.

[0005] A solution to this problem can be achieved by mixing thechemically active compound with a plastisol or other matrix formingmaterial and curing it with the active compound in particulate or powderform substantially evenly distributed throughout its mass. Generally theplastisol mixture is applied as a layer to a sheet forming a substrateand is covered by another sheet to form a three-layered laminate. Bothsheets are pervious to moisture and sulphur dioxide gas. By ensuringadequate but controlled access by moisture in the surroundings to thechemical compound within the plastisol mass (i.e. in the plastic bag inactual use) the quantity of sulphur dioxide produced, and the rate atwhich it is produced, can be controlled to an advantageous extent.

[0006] Control is achieved by selecting the various variables includingthe thickness of the layer of mixture; the amount of chemical compoundmixed with the plastisol; and the means employed for enabling moistureto penetrate the cured plastisol mass at a rate commensurate with therequired rate of production of sulphur dioxide over a required period oftime. Clearly there also needs to be the facility for enabling sulphurdioxide generated within the cured plastisol mass to migrate from themass to the atmosphere in the packaging bag in order that it may carryout its preservative action. Such facility will generally be inherentlypresent but it may also require separate consideration.

[0007] The said means for enabling moisture to penetrate the plastisolmass can be the provision of permeable particles in the mass whichsubstantially touch each other and thus form a path for moisture toenter the mass. Alternatively, or in addition, small gas bubbles can beeither generated within the plastisol during the curing process by usinga blowing agent or the like, or air may be mixed into the plastisolduring the blending of the plastisol and other ingredients together. Inthe latter case the gas is typically air. A humectant may also be mixedinto the plastisol mass in order to attract moisture to the interiorthereof. The resultant laminate is then cut up into pieces of a sizesuitable for inclusion in a package. A composition of this general typeforms the basis of South African patent number 96/2517.

[0008] However, production of the laminate is difficult to control and,in addition, the cut pieces of laminate may have particles of chemicalcompound at the cut surface. The latter may have a deleterious effect onfruit which comes into direct contact with such a cut edge. Exposedchemical compound at the cut edges may also be objectionable from ahealth point of view.

[0009] For this reason, the laminate is sometimes placed in amoisture-permeable sleeve which is then included in the fruit package,but this increases the production costs and it is also time consuming toindividually insert each laminate into a sleeve.

[0010] Accordingly, a need exists to provide an in-package preservativegas generating device which does not exhibit the disadvantages outlinedabove, at least to the same extent and which can be manufactured in anexpeditious manner.

SUMMARY OF THE INVENTION

[0011] In accordance with one aspect of this invention there is providedan in package preservative gas generating device comprising a layer of amatrix forming material having a preservative gas generating compounddispersed therein adhered to a carrier sheet in the form of a layercovering a central area of the carrier sheet so as to form at least onediscrete zone with the peripheral area of the carrier sheetsubstantially free of such mixture and a cover sheet secured to saidperipheral area to enclose the layer of mixture between the carriersheet and the cover sheet, the layer of matrix being adapted to permitthe generation of a preservative gas under operative conditions by thegas generating compound at a controlled rate and to permit it topermeate out of the said matrix, and wherein at least one of the carriersheet and the cover sheet are permeable to said preservative gas.

[0012] Further features of the invention provide for the preservativegas generating compound to be a moisture activated compound in whichcase the layer of matrix is adapted to permit the ingress of moisture ata controlled rate; for the matrix forming material to be a plastisol;for the plastisol to be formed with a multitude of small gas bubblesentrained therein; for the carrier sheet to have a thermoplastic surfaceto which the cover sheet is thermally bonded such as, for example, aplastic coating typically of polyethylene carried a support sheet whichmay be in the form of a more temperature resistant plastics film such asa polyester film or a suitable paper in which case the carrier sheet canbe impermeable to both moisture and the preservative gas; and for thecover sheet to be a permeable sheet such as, for example, a non-wovenfibrous plastic sheet, typically a non-woven polyester fabric.

[0013] In accordance with a second aspect of the invention there isprovided a method of manufacturing an in-package preservative gasgenerating device as defined above including the steps of applying alayer of a matrix forming material having a preservative gas generatingcompound dispersed therein to an extended carrier sheet with the layerbeing arranged in discrete zones thereof spaced apart from each otherand each of which is surrounded by an area of carrier sheet materialdevoid of such matrix; treating the matrix as necessary in order to cureit, dry it, or otherwise stabilize it; covering the extended sheet witha cover sheet; causing the cover sheet to become bonded to the carriersheet in the said areas devoid of said mixture, and subdividing theextended carrier sheet and associated cover sheet to form in packagepreservative gas generating devices containing one or more of saiddiscrete zones.

[0014] Further features of this aspect of the invention provide for themixture to be as defined herein above; for the mixture to be applied tothe carrier sheet by an extrusion process in which a layer of mixture isurged under positive pressure out through a nozzle onto a travellingextended carrier sheet, the nozzle conveniently being of a widthapproximately equal to the width of the said discrete zones and whereinthe spacing of the nozzle from the travelling carrier sheet, at least inpart, determines the thickness of the layer; for the carrier sheet tohave a thermoplastic surface typically formed as a thermoplastic layerbonded to a support sheet which may be in the form of a more temperatureresistant plastics film or a suitable paper- for the matrix formingmaterial to be a plastisol in which case curing thereof is effected byheating, and for the preservative gas generating compound to be adaptedto form sulphur dioxide on contact with moisture.

[0015] In its most preferred form the method of the invention is carriedout on a substantially continuous basis by extruding a layer of saidmixture in discrete zones onto a travelling extended carrier sheet whichis then caused to travel past a heating assembly in order to cure thematrix forming material following which thermal bonding of the coversheet to the carrier sheet is performed along longitudinal andtransverse strips between the said discrete zones.

[0016] In order that the invention may be more fully understood, oneembodiment thereof will now be described with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the drawings:

[0018]FIG. 1 is a partly broken away isometric view of a singlein-package preservative gas generating device according to theinvention; and

[0019]FIG. 2 is a schematic illustration of the process whereby the gasgenerating device illustrated in FIG. 1 is produced.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

[0020] In this embodiment of the invention an in-package sulphur dioxidegenerating device comprises a carrier sheet (1) which in this particularcase is a polyester film having on one surface thereof a layer or filmof a thermoplastic material more appropriate to lower temperaturethermal welding such as polyethylene, for example. The exactconstruction of the carrier sheet will depend on requirements. The sheetcould, for example, be a co-extruded sheet.

[0021] Alternatively, a polyester film may be printed with any one ormore of identifying informative and decorative material and thereafterhave a film of polyethylene laminated thereto in known manner in thepackaging industry. In either event the construction renders the carriersheet impermeable to both moisture and sulphur dioxide gas. Asindicated, the polyethylene layer renders that surface, which isdirected operatively inwardly (i.e. upwardly in the illustratedorientation), susceptible to thermal welding.

[0022] A substantially rectangular discrete zone (2) of a layer of amatrix forming material, in this case in the form of a PVC plastisol,and a sulphur dioxide generating compound in the form of sodiummetabisulphite, is adhered to the carrier sheet such that an endlessperipheral area (3) is left devoid of any mixture. This matrix mixtureis more fully described below.

[0023] A cover sheet (4), in this case made of a non-woven fibrouspolyester fabric, covers the discrete zone of cured matrix mixture andis thermally bonded to the polyethylene layer or the carrier sheetaround the entire periphery as indicated by numeral (5). The bond is, inthis case, more a mechanical bond than a true welded fusion bond. As analternative to the above fabric, a spun bonded polypropylene orpolyethylene fabric could be used as the permeable cover sheet.

[0024] In use, the in-package sulphur dioxide generating device can beused in the usual way by installing it on top of the fruit (orvegetables) in a plastic bag, usually in a cardboard box. The generatingdevice is orientated with the cover sheet directed downwards and towardsthe fruit so that moisture can be received through it to migrate intothe matrix to activate the sulphur dioxide generating compound in acontrolled manner over a pre-designed period of time. Clearly, there isno wastage of sulphur dioxide gas as the impermeable carrier sheet isdirected outwardly and the permeable cover sheet allows the gasgenerated to pass directly to the fruit. Also, there is littlepossibility of any part of the layer of mixture coming into contact withthe fruit directly or, for that matter, with any part of a personhandling the sulphur dioxide generating device or the fruit.

[0025] The SO₂ gas will generally only be generated at relatively highhumidity levels within the package, for example, at humidity levels ofbetween 85 and 100%. The rate at which the SO₂ gas is released from thedevice can be varied according to specific requirements, and generallythe rate is either a “constant” release rate or a “sharp peak” releaserate. If the device is configured to generate a constant release rate,then the SO₂ generally begins to be released within about 12 hours ofpackaging, and increases until-the SO₂ concentration within the packageis approximately 30 ppm. Thereafter, the SO₂ concentration willgradually decrease to between 5 to 15 ppm over a six to eight weekperiod. If, however, the device is configured to produce a “sharp peak”release rate, then the SO₂ concentration within the package will rise toabout 70 to 80 ppm within 1 to 2 days, and will thereafter decrease to 5to 10 ppm within a few days, and this lower concentration can bemaintained for six to eight weeks.

[0026] Turning now to the production of the sulphur dioxide generatingdevice described above, and to the composition of the matrix layer, theexact configuration and matrix mixture will depend on requirements, inparticular, the targeted shelf or storage life of the fruit concerned,the nature of the fruit, the cost allowed for the sulphur dioxidegenerating device, and any other considerations.

[0027] Simply by way of example of a plastisol which has proved to besuccessful in the packaging of grapes the following is given as apreferred formula: PVC powder having a K value of 80: 58.0% Epoxidizedsoya bean oil (ESBO) Plasticizer 40.0% (available in South Africa fromChemserve Ltd) Stablizer in the form of Zinc Stearate in ESBO  1.0% ascarrier (available from Chemserve Ltd) Blowing Agent being Tracel DB145NER  1.0% (an azodicarbonamid mixture from Tramanco of PinneburgGermany)

[0028] To this was added sodium metabisulphite particles having aparticle size of <100 μm in an amount of 0.2 kg (variable from 0.1 to0.3 kg according to the particular application) per kg of the aboveplastisol mixture. The matrix mixture was blended under a vacuum in ablender.

[0029] The in-package sulphur dioxide generating devices described aboveare preferably made on a substantially continuous basis using anextended carrier sheet in roll form as indicated by numeral (6) in FIG.2, the carrier sheet being transported on a support system through aseries of items of processing equipment.

[0030] The first of these is an extrusion device (7) which is adapted toextrude matrix mixture through a series of nozzles in the form of slotsextending across the width of the device. The extruded layer of matrixis discontinuous so that rectangular discrete zones (8) of the matrixare deposited on the travelling carrier sheet.

[0031] The sheet is then caused to pass under a series of infraredheaters (9) in order to cure the plastisol.

[0032] Thereafter, cover sheet material from a roll (10) thereof isapplied over the carrier sheet with the discrete zones of cured matrixmixture thereon.

[0033] A welding station (11) causes the cover sheet to become thermallybonded to the plasticized surface of the carrier sheet along transverseand longitudinal strips.

[0034] The final station (12) is a cutting and slitting station whichsubdivides the substantially continuous web of interconnected sulphurdioxide generating devices as described above into the required sizes.

[0035] It will be understood that numerous variations may be made to theembodiment of the invention described above without departing from thescope hereof. In particular, the formula employed for making the matrixcan be varied widely as can the method of producing the in-packagepreservative gas generating devices. Also, the invention is not to beinterpreted as being limited to sulphur dioxide as the preservative gasand any other preservative gas capable of in situ generation could beused.

1. A preservative gas generating device including: a carrier sheet; alayer of a matrix forming material having a preservative gas generatingcompound dispersed therein, the layer of matrix forming material beingadapted to permit the generation of a preservative gas under operativeconditions by the preservative gas generating compound at a controlledrate and to permit the gas to permeate out of the matrix formed by thematerial, the matrix forming material being adhered to the carrier sheetin the form of a layer covering an area of the carrier sheet so as toform at least one discrete zone with a peripheral area of the carriersheet surrounding the discrete zone(s) being substantially free of thematrix forming material; and a cover sheet secured to said peripheralarea to enclose the layer of matrix forming material between the carriersheet and the cover sheet; and wherein at least one of the carrier sheetand the cover sheet are permeable to said preservative gas.
 2. A gasgenerating device according to claim 1, wherein the preservative gasgenerating compound is a moisture activated compound and the matrix isadapted to permit the ingress of moisture at a controlled rate.
 3. A gasgenerating device according to either of claims 1 or 2, wherein thematrix forming material is a plastisol.
 4. A gas generating deviceaccording to claim 3, wherein the plastisol is formed with a multitudeof gas bubbles entrained therein.
 5. A gas generating device accordingto any one of claims 1 to 4, wherein the carrier sheet has athermoplastic surface to which the cover sheet is thermally bonded.
 6. Agas generating device according to claim 5, wherein the thermoplasticsurface is a plastic coating of polyethylene carried on a support sheetof a temperature resistant plastics film or a suitable paper, andwherein the carrier sheet is impermeable to both moisture and thepreservative gas and the cover sheet is a permeable sheet.
 7. A gasgenerating device according to claim 6, wherein the cover sheet is anon-woven fibrous plastic sheet.
 8. A gas generating device according toany one of claims 1 to 7, wherein the preservative gas is intended to beformed under humidity conditions of 85 to 100%.
 9. A gas generatingdevice according to any one of claims 1 to 8, wherein the preservativegas generating compound forms sulphur dioxide on contact with moisture.10. A gas generating device according to any one of the previous claims,which includes a plurality of discrete zones of matrix forming material,each zone being surrounded by a peripheral area which is substantiallyfree of the matrix forming material.
 11. A method of manufacturing apreservative gas generating device according to any one of claims 1 to10, which includes the steps of: applying a layer of a matrix formingmaterial having a preservative gas generating compound dispersed thereinto an extended carrier sheet, with the layer being arranged in discretezones spaced apart from each other and each of which is surrounded by anarea of carrier sheet material substantially devoid of such matrix;optionally treating the matrix forming material in order to cure it, dryit, or otherwise stabilize it; covering the extended sheet with a oversheet; causing the cover sheet to become bonded to the carrier sheet inthe said areas devoid of the matrix forming material; and optionallysubdividing the extended carrier sheet and associated cover sheet toform in-package preservative gas generating devices containing one ormore of said discrete zones, each device having a peripheral areasubstantially free of the matrix on its outer edges.
 12. A methodaccording to claim 11, wherein the preservative gas generating compoundforms sulphur dioxide on contact with moisture.
 13. A method accordingto either of claims 11 or 12, wherein the matrix forming material isapplied to the carrier sheet by an extrusion process in which a layer ofthe material is extruded through at least one nozzle onto a travellingextended carrier sheet, the nozzle(s) being of a width approximatelyequal to the width of the said discrete zones and wherein the spacing ofthe nozzle(s) from the travelling carrier sheet, at least in part,determines the thickness of the layer.
 14. A method according to any oneof claims 11 to 13, wherein the carrier sheet has a thermoplasticsurface formed as a thermoplastic layer bonded to a support sheet; andthe cover sheet is manufactured from a moisture and gas permeablematerial.
 15. A method according to claim 14, wherein the support sheetis a temperature resistant plastics film or a suitable paper.
 16. Amethod according to any one of claims 11 to 15, wherein the matrixforming material is a plastisol which is cured by heating.
 17. A methodaccording to any one of claims 11 to 16, which is carried out on asubstantially continuous basis by extruding a layer of said matrixforming material in discrete zones onto a travelling extended carriersheet which is then passed through a heating assembly in order to curethe matrix forming material, following which thermal bonding of thecover sheet to the carrier sheet is performed along longitudinal andtransverse strips between the said discrete zones.
 18. A preservativegas generating device substantially as herein described and illustrated.19. A method of manufacturing a preservative gas generating devicesubstantially as herein described and illustrated.